Can anyone tells me what this means?

I have been dating this guy for almost a year and he has a best friend with a girl. At first I was super cool with it but now he spends way more time with her, like 5 hours every other day and barely ever want s to see me. Every time I confront him about he tells me I shouldn't worry because he doesn't see her like that and that they are just friends. Should I believe him, what should I do?
Can anyone tells me what this means?
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